The present invention refers to a process for the manufacturing of ceramic-matrix composite layers resistant to very high temperatures, like e.g. Boride mixtures, like e.g. Zirconium, Titanium and/or Hafnium Borides, or Oxide mixtures, like e.g. Hafnium and/or Zirconium Oxides, and Carbides, like e.g. Silicon, Tantalum, Zirconium and/or Hafnium Carbides. These layers can serve as protective coating of the material onto which they are coated, or they can be employed as components per se, upon having removed the substrate material, e.g. by machining or by chemical attack. Hence, the present invention further refers to the composite material obtained by said process.
Of late years, a specific class of materials, that of the Ultra High Temperature Ceramics (UHTC) has aroused a marked interest for prospective employs thereof in the aerospace field.
In particular, a group of composites having as matrix the Borides of some transition metals (ZrB2 and HfB2) is deemed to be the most suitable to the thermal protection of sharp leading edges for the new generations of space vehicles, apt to reenter Earth atmosphere after a stay thereof in outer space and to be launched again therein.
The context of said viable and exemplary employ of the materials object of the present invention will be detailed hereinafter.
The material specifications required by such a specific employ are extremely complex and difficult to attain all in the same material. In fact, the materials at issue should combine high thermal conductivity coefficients and high resistance to thermal shocks to a very high melting point and a low thermal expansion coefficient.
The high melting point, apt to reach the 3300 K, is a real obstacle to the shaping of these materials. In fact, the sole technique capable of ensuring satisfactory features in terms of porosity and of mechanical resistance is the hot pressing, entailing, however, remarkable limitations in terms of sizes and costs.
Other shaping techniques are known. E.g., the plasma thermal spraying overcomes the problems entailed in the hot pressing forming techniques of the state of the art. In fact, the employ of a plasma that in its hotter region could reach and exceed the 20000 K enables the deposition of virtually all those elements or compounds having a stable liquid phase. However, it should be borne in mind that the manufacturing of self-standing, high-melting ceramics-based coatings and supports still constitutes, due to the required powers, a frontier research field in the employ of the plasma thermal spraying. Moreover, depositing a composite material entails the risk of interactions between the constituent phases thereof.
The present invention consists of a methodology of depositing by plasma thermal spraying, yielding Boron (Boride)-based, Oxygen (Oxide)-based, Boride and Oxide mixtures-based, and Carbon (Carbide)-based ceramic-matrix composite materials. These composites are characterized by the concomitant presence of highly compact ceramics phases (density  greater than 96% of the theoretical density).
The present invention aims at attaining these features, overcoming the obstacles present in the state of the art.
Hence, the present invention refers to a process as abovespecified, comprising the steps of:
preparing the powders for the feeding of the deposition plant by wet mixing the ceramics constituting the material in form of fine powders and atomizing the suspension in the presence of a hot air jet;
depositing by plasma thermal spraying with an inert gas flow and with a  greater than 30 kPa pressure in a deposition chamber.
The main advantage of the process according to the invention lies in forming complex-shaped and self-standing composite layers suitable to the most extreme employs, as in the coatings of space vehicles.
The present invention will hereinafter be described according to preferred embodiments thereof, given as a non-limiting example with reference to examples reported hereinafter and to the individual FIGURE attached.